1. Field of the Invention
The present invention relates to an electric double layer capacitor and a method for preparing the same.
2. Description of the Prior Art
Heretofore, electric double layer capacitors have been put to practical use as capacitors which enable obtaining high capacities of farad(F)-order without involving chemical reaction and which endure charge and discharge of heavy currents and charge-and-discharge cycles.
In recent years, with a view to making good use of electric double layer capacitors, studies on new applications thereof as an auxiliary power supply of an electronic appliance such as a cellular phone. Concomitantly therewith, further miniaturization and capacity increase of electric double layer capacitors have been attempted.
As shown in FIG. 4, a conventional electric double layer capacitor comprises a basic cell 10 including a porous separator 11 in the form of a sheet; a pair of current collectors in the form of a sheet disposed oppositely relative to the separator 11; a pair of polarizable electrodes 13, 13 having a plate-like form and disposed between the separator and the current collectors; and gasket 14 which is laterally contiguous to the separator 11 and the polarizable electrodes 13, 13 and held between the current collectors 12, 12. The basic cell 10 is sealed with an electrolytic solution contained therein.
In most cases, the gasket 14 is prepared from two gasket elements 141, 141 for manufactural reason.
In the next place, a method for preparing a conventional electric double layer capacitor will be described with reference to FIG. 4.
FIGS. 4(a) to 4(g) are sectional views schematically showing a part of a method for preparing a conventional electric double layer capacitor.
As shown in FIG. 4(a), a current collector 12 in the form of sheet is provided and, as shown in FIG. 4(b), it is cut down to size.
As the current collector 12, an electrically conductive film is generally employed which comprises an electrical insulating resin and electrically conductive particles. With respect to material and method for the production thereof, there maybe employed conventional techniques, for example, disclosed in Japanese Unexamined Patent Publication Nos. 1998-4033 and 1998-4034 by Nagaki et al.
Further, as disclosed in U.S. Pat. No. 6,377,441 by Ohya et al., a laminate prepared by laminating a plurality of current collectors 12 may be employed.
Moreover, as materials suitable for the current collector, those disclosed in PCT/JP98/01021 and PCT/JP98/03073 may be employed.
Then, as shown in FIG. 4(c), a gasket element 141 in the form of a frame, i.e., a gasket element 141 having an opening is disposed on the surface of the current collector 12. Since the gasket element 141 is disposed on a peripheral portion of the surface of the current collector 12, a portion of the surface of the current collector 12 within the opening remains uncovered.
Then, as shown in FIG. 4(d), a polarizable electrode 13 is provided on the uncovered portion of the surface of the current collector 12 and, as shown in FIG. 4(e), a separator 11 is so disposed on the gasket element 141 as to cover the opening of the gasket element 141.
Thereafter, as shown in FIG. 4(f), another product as shown in FIG. 4(d) is disposed on the separator 11 in such a manner that inner portions of the gasket elements 141, 141 of the two products as shown in FIG. 4(d) oppositely abut on the separator 11, and the gasket elements 141, 141 are subjected to thermo-compression bonding from both sides via the current collectors 12, 12.
The gasket elements 141, 141 are deformed so that outer portions of the gasket elements 141, 141 vertically extend all around the separator 11, and the gasket elements 141, 141 are fusion-bonded together into a gasket 14. As a result, the separator 11 is laterally covered with the gasket 14 all around to thereby obtain an electric double layer capacitor as shown in FIG. 4(g).
In recent years, miniaturization of electronic parts has been advanced. In particular, the advents of small-sized electric double layer capacitors having high capacities and the like have strongly been desired as described above.
However, according to the conventional method for preparing an electric double layer capacitor, if it has been intended to prepare an electric double layer capacitor as a small-sized capacitor with a high capacitor, there have been limitations in size and in mechanical strength which are attributed to the preparation method.
Specifically, a current collector used in a conventional electric double layer capacitor has been a butyl rubber containing a carbon powder in most cases. With respect also to a gasket, a butyl rubber has been used because of its affinity with the current collector.
If a current collector and a gasket which are made of a butyl rubber or the like have been miniaturized in order to advance miniaturization of electric double layer capacitors, there has been a problem in poor mechanical strength in the fusion-bonding step or the like. This has created a limitation on miniaturization of electric double layer capacitors.
Further, if each of members including a current collector has been reduced to too thin a film with an intention to miniaturize electric double layer capacitors, transmission of electrolytic solution vaporized in the step of thermo-compression bonding has been liable to occur. As a result, there have been undesirable variations in equivalent series resistance (ESR). This leads to a lowered yield of electric double layer capacitors as products.